A. Field of the Invention
The present invention relates to the reinforcement of ventilating ducts of the type used to convey air for heating, ventilating or air conditioning (HVAC) the interior space of a building. More particularly, the invention relates to an improved tie rod fastener for preventing excessive flexure of duct walls in response to fluctuations of air pressure exerted on walls of the duct.
B. Description of Background Art
Ductwork used to heat, ventilate, or air condition buildings usually consists of elongated lengths of tubing comprising individual duct sections which are coupled together to form a continuous, air-tight duct for conveying flowing air. Typical duct tubing is made of a material such as sheet metal which is relatively strong but sufficiently light in weight to minimize the size and weight of fasteners and structural components required to support the ductwork, as well as maintaining materials and fabrication costs of the ductwork itself at reasonable levels. Thus, typical ducts are made of relatively thin sheet steel ranging in size from 26 gauge (0.0188 inch thick) to 16 gauge (0.0625 inch thick).
Sheet metal ducts have rectangular, circular or oval cross section shapes, and are often manufactured and supplied in pre-cut lengths or sections with transversely outwardly protruding interconnection flanges provided at opposite longitudinal ends of the section, to facilitate interconnecting duct sections at a job site and thus forming air conveying ducts of desired lengths and orientations.
Ducts of the type described above are available in a wide variety sizes of, having cross-section dimensions which range from a few inches to several feet. Moreover, a wide range of width-to-height or aspect ratios of rectangular ducts are available. For example, a typical 18 inch high duct may have a width in the range of 2 feet to 4 feet, and a length of 5 feet. Whatever the shape, size and aspect ratio of the duct, the relatively small thickness of its walls relative to its cross-sectional dimensions results in the duct walls being relatively flexible. Thus, conventional ducts may experience relatively large, possibly destructive deformations if static or dynamic differential air pressure between the interior and exterior of the duct exceeds pre-determined threshold values. For this reason, mechanical engineering standards as well as applicable building codes require that duct work used to conduct air in certain heating, ventilating, and air conditioning (HVAC) applications be reinforced against expansion when positively pressurized and/or against collapse when negatively pressurized.
A widely employed reinforcement method that meets code requirements consist of installing elongated straight, rigid reinforcement members within a duct at pre-determined spacings which depend upon the rigidity of the duct, and upon the maximum differential pressures which it may encounter. Such reinforcement members are disposed perpendicularly between inner surfaces of opposite walls of the duct and fastened at opposite ends thereof to those walls. One such approved reinforcement member which is in current use consists of a threaded steel tie rod which has installed onto each end thereof a first, inner nut which is threadingly advanced to a predetermined distance inward from the end of the rod. The distance between the outer faces of the inner nuts is made equal to the minimum cross-sectional dimension, i.e., the height of the duct. The tie rod is next positioned perpendicularly between a pair of opposed walls of the duct, and opposite ends of the threaded rod are each inserted into a separate one of a pair of transversely or vertically aligned holes provided through the duct walls. A pair of external nuts are then threaded onto the opposite ends of the threaded rod which protrude through the duct wall holes. Each external nut is then tightened onto the threaded rod against the outer surface of the duct wall, while the adjacent inner nut is held against rotation with a separate wrench. For large ducts, this operation requires two workmen.
Another prior-art duct reinforcement uses an elongated tube containing in opposite ends of the bore openings thereof an internally threaded fastener member which is fixed in the tube and which is adapted to receive a machine screw inserted through a duct wall hole. This reinforcement method also sometimes requires that the tube being gripped while the external machine screw is torqued, which again may require two workmen.
A third type of prior art tie rod used to reinforce HVAC ducts, which is a variation of the first method, uses a tube which encloses a threaded rod, the latter being secured between the walls of the duct by two external nuts, and is no easier to install than the other two types described above.
In addition to being somewhat difficult and time consuming to install, in accordance with certain code requirements, prior art reinforcement tie rods of the type discussed above must make an air-tight seal with the duct wall holes through which they protrude, thus requiring installation of a resilient sealing element such as a gasket or washer between the exterior nut or screw head and the duct wall. If the exterior nut or screw is torqued too tightly, such resilient elements can be deformed or damaged, and may ultimately fail to achieve sealing. Patents known to the present inventor which are related to reinforcement or sealing members that may have applicability to the field of the present invention include the following United States patents:
Augustin, U.S. Pat. No. 3,009,722, Nov. 21, 1961, Sealing And Retaining Ring PA1 Jones et al., U.S. Pat. No. 3,343,440, Oct. 18, 1965, Self-Locking Two-Piece Fastening Device PA1 Savage, U.S. Pat. No. 3,557,838, Jan. 26, 1971, Duct Stiffener PA1 Yonkers, U.S. Pat. No. 3,606,357, Sep. 20, 1972, Self-Sealing And Aligning Member PA1 Yamaguchi et al., U.S. Pat. No. 3,776,253, Dec. 4, 1973, Means For Preventing Deformation Of Steel Tubes PA1 Puklus, Jr., U.S. Pat. No. 3,878,757, Apr. 22, 1975, Automatic Locking Sliding Nut PA1 Freeman, U.S. Pat. No. 4,249,578, Feb. 22, 1981, Length-Adjustable Stiffener For Fiberboard Ducts PA1 Kowalski, U.S. Pat. No. 4,634,327, Jan. 6, 1987, Fastening Device For Attachment To A Threaded Rod PA1 Arav, U.S. Pat. No. 4,671,546, Jun. 9, 1987, Sealing Device For Securing Device Extending Through Fluid Container PA1 Hunter, U.S. Pat. No. 5,253,901, Oct. 19, 1993, Duct Reinforcement PA1 Elder, U.S. Pat. No. 5,660,212, Aug. 26, 1997, Integral HVAC Reinforced Duct System And Method For Reinforcing Duct
None of the foregoing references discloses or suggests structures or methods which might overcome the limitations of prior art tie rod fasteners of the type discussed above. In view of the aforementioned limitations of existing tie rod fasteners, the present convention was conceived of.